Burning the toast
Stories from Physics for 11-14 14-16
It is challenging to toast bread to the perfect colour because of thermal runaway: as the surface of the bread darkens it absorbs more thermal radiation so its temperature rises faster and the blackening process accelerates. A more serious but related effect occurs as highly reflective white ice sheets melt, decreasing the net reflectivity of the Earth’s surface and accelerating global warming. In reverse, this phenomenon can be used to make urban environments more comfortable. As part of the city’s urban cooling agenda, a pilot programme in Los Angeles has painted some normally dark-coloured pavements with a white, reflective paint. The paint led to a decrease in pavement surface temperatures by up to 6°C.